Bacteria – Morphology &
Classification
Introduction:
• Microorganisms – several classes of living beings
• Based on the organization of their cellular structures,
all living cells can be divided into two groups:
eukaryotic and prokaryotic
– Eukaryotic cell types - Animals, plants, fungi,
protozoans, and algae
– Prokaryotic cell types - bacteria & blue green algae
Structure
Prokaryotes
Eukaryotes
Nuclear membrane
Absent
Present
Nucleolus
Absent
Present
One
More than one
Division
Binary fission
Mitosis
Cytoplasm
Cell organelles
absent
Present
Absent
Present
Nucleus
Chromosome
Chemical
composition
Sterols
• Bacteria are protista
• Kingdom protista is the 3rd major branch of
micro-organisms that includes:
- Higher protista (eukaryotic)
1- protozoa
2- fungi
3- Algae except blue green algae
- Lower protista (prokaryotic)
1- bacteria
2- blue green algae
CLASSIFICATION
I- based on morphology (old)
• 1- Higher bacteria
Actinomyces
(filamentous
branching
Leptospira
(filamentous non
branching)
• 2- Lower bacteria
- Simple unicellular
- Classified acc to
morphology into:
Cocci Bacilli
curved spiral
filaments
Lower Bacteria
curved
bacilli
Higher Bacteria
cocci
Spiral
Actinomyces
Arrangement of bacteria: Cocci
Coccus
Tetrad – groups of four
Cocci in pair –
Diplococcus
Cocci in chain - Streptococci
Cocci in cluster - Staphylococci
Sarcina – groups of eight
Arrangement of bacteria: Bacilli
Bacterial Cell Structure
Essential components: • Non essential
components:
basic structure
present in some
present in all
bacteria species, e.g.
bacteria, e.g. cell
capsule, fimbria,
wall, Cytoplasmic
flagella, spores
membrane,
cytoplasm, nuclear
material.
Cell Wall
• The cell wall is the outermost component of
bacterial cell.
• Functions:
1- maintain shape
2- support cytoplasmic membrane
3- Role in cell division
4- protection against antibiotics & antibodies
5- Antigenicity
6- Staining reaction to Gram
Composition of cell wall
I- Gram positive cell wall
Two layers
1- Peptidoglycan:
constitute 80% of cell wall
thickness.
Mucopeptide chain
composed of alternate
units of N-Acetyl
Muramic acid and NAcetyl Glucosamine
linked by tetrapeptide
chains
2- Teichoic acid
It is located in the outer
layer of the Gram
positive cell wall. It is
antigenic.
2- Gram negative cell wall
•
Peptidoglycan: constitutes up to 5-10% of Gram
negative cell wall.
•
Outer membrane: formed of bilayered
phospholipids It has special channels, consisting of
proteins called porins which allow passive diffusion
of low molecular weight compounds like sugar,
and amino acids.
•
Periplasmic space: a space between the inner
cytoplasmic membrane and outer membrane
where the single sheet of peptidoglycan layer is
present. contains gel-like solution of protein.
G+ve
G-ve
Summary of the differences between Gram
positive & Gram negative bacteria
Property of bacteria
Gram
Positive
Thickness of wall
Number of layers in wall
Peptidoglycan content
Teichoic acid in wall
Lipid & lipoprotein content
Protein content
Lipopolysaccharide
Sensitive to penicillin
Digested by lysozyme
20-80 nm
1
>50%
+
0-3%
0%
0
Yes
Yes
Gram
Negative
10 nm
2
10-20%
58%
9%
13%
Less sensitive
Weakly
Cytoplasmic Membrane
• It lies just inside the peptidoglycan layer. It is a
phospholipids bilayer that contains protein. It is
similar to eukaryotic cell membrane but does not
contain sterol except in mycoplasma.
Mesosomes are invagination of the cytoplasmic
membrane inside the cytoplasm.
Function of Cell Membrane:
• Selective permeability: Molecules move
across the membrane by simple diffusion
or active transport.
• Excretion of extracellular enzymes and
toxins.
• Respiration: the cytochrome and
respiratory enzymes are localized in the
cytoplasmic membrane.
• Play an important role in cell division.
Cytoplasm
 The cytoplasm of the bacterial cell is a viscous
watery solution or soft gel that contains a variety
of organic and inorganic solutes.
A- Mesosomes:
 These are invagination of cytoplasmic membrane.
 It is the site of attachment of DNA chromosome
during cell division.
 Site of respiratory activity of the cell.
 Increase surface area of the membrane, thus
increase efficiency of active transport.
B- Ribosomes:
• They are complex structure composed of
60% RNA and 40% protein. They are the
site of protein synthesis in the cell.
• They have sedimentation constant of 70S
being composed of 30S and 50S subunits.
C- Inclusion granules:
• Round granules observed in cytoplasm of
many bacteria.
• These are not permanent or essential
structures.
• They appear to be either stored energy or
nutrient reserve concerned with cell
metabolism, e.g. volutin granules
(Metachromatic granules of
Corynebacterium diphtheria)
E- The Nuclear Body:
- The genetic information of the bacterial
cell is contained in single circular DS-DNA
molecules, which constitutes the bacterial
chromosome.
- The nuclear body does not have nuclear
membrane, mitotic apparatus, or histones.
Flagella
• Flagellae are the organs of motility. They are
thread like appendages, protein in nature,
formed of flagellin protein which is antigenic (H
Ag).
• They can not be stained by gram stain. They
have special stain.
• According to their number, they may be
monotrichate, amphitrichate, lophotrichate,
peritrichate.
FLAGELLA: Functions
1. Locomotory organelles- flagella
2. Taste their environment
3. Respond to specific chemical foodstuffs
or toxic materials and move towards
or away from them (chemotaxis)
Each flagellum has three parts:
1. Filament
2. Hook
3. Basal body
Arrangement types
a. Monotrichous: Single polar flagellum
b. Amphitrichous: Single flagellum at both
ends
c. Lophotrichous: Tuft of flagella at one or
both ends
d. Peritrichous: Flagella arranged all around
cell
Monotrichious
Lophotrichious
Peritrichious
Demonstration of Flagella
1.
2.
3.
4.
Dark ground illumination
Electron microscopy
Special staining
Indirect methods
a. Motility
b. spreading growth
c. Craigie’s tube
Fimbriae (pili)
• They are a hair like filaments,
formed of protein called pillin
which is antigenic in nature.
Fimbriae are responsible for the
attachment of bacteria to specific
receptors of human cell
(Adherence).
•
• There are special types of pili
called sex pili involved in the
process of conjugation (transfer of
DNA between bacteria).
•
• Fimbriae are shorter and thinner
than flagella.
Structures outside the Cell Wall
Capsule
• It is present only in some bacteria outside the cell wall.
It is gelatinous in nature. The capsule may be
polysaccharide, polypeptide or hyalouronic acid
• Capsules usually formed in vivo only and not stained
by ordinary stain. The capsule is antigenic.
• The capsule has antiphagocytic function so it
determine the virulence of many bacteria. It also plays
a role in attachment of the organism to mucous
membrane
Demonstration of Capsule
• Stained by negative
staining using India
Ink
• Can be demonstrated
by Quellung reaction
(capsule swelling
reaction)
Bacterial Spores (Endospores)
• Some bacterial genera are capable of forming
highly resistant resting phase or endospores, e.g.
Bacillus group and Clostridium group.
• Endospores do not reproduce and exhibit
absolute dormancy. Spore formation occurs in
response to unfavourable conditions, e.g.
depletion of nutrition, accumulation of
metabolites or unsuitable temperature or
moisture.
• When the unsuitable conditions changed, the
spore germinates to the vegetative form which
can multiply.
STRUCTURE OF BACTERIAL ENDOSPORE
Shape & position of bacterial spore
Oval central
Spherical central
Non bulging
Oval sub terminal
Oval sub terminal
Oval terminal
Spherical terminal
Free spore
Bulging
Demonstration of Spores
• Unstained preparations: Seen as refractile
bodies
• Grams stain: seen as unstained areas
• Modified Ziehl-Neelsen stain: 0.25 %
Sulphuric acid.
Importance of Spores
• Helps bacteria to survive for longer periods under
unfavourable conditions.
• Act as mode of transmission.
• Sterilization controls: Spores of all medically
important species are killed by autoclaving at
121⁰C for 15 min. Spores are killed in autoclave if
above temperature is attained.
Eg: Bacillus stearothermophilus, Bacillus subtilis
Pleomorphism and Involution
forms
• Pleomorphism: Variation in shape and size
of individual cells.
• Involution forms: Aberrant and swollen
forms in ageing (old) cultures.
L-Forms
• Swollen and other aberrant morphological
forms.
• Named as L-forms after Lister institute
London.
• Develop either spontaneously from bacteria
or in presence of agents destroying cell wall
like Penicillin.
• L-forms may be stable or unstable